The present invention relates to an apparatus for image formation which fixes a developer image on a record medium by heating and pressing the medium.
A copier, a laser printer, or another apparatus for electrophotographic image formation includes a heater, a fixing member, and a pressing member. The heater heats the fixing member. The pressing member is in compressive contact with the fixing member. The fixing and pressing members fix a developer image on a record medium by heating and pressing the medium passing through the nip between these members.
An apparatus for electrophotographic image formation includes a heater, a fixing member, a pressing member, a temperature sensor, and a controller. The temperature sensor senses the surface temperature of the fixing member. The controller adjusts the surface temperature to a specified range by controlling the current supply to the heater according to the sensed temperature.
Part of the paper dust, developer dust, or other dust on the fixing member may stick to the temperature sensor. If dust sticks to the temperature sensor, heat is conducted from the fixing member to the sensor through the dust. The dust on the temperature sensor changes the condition of the sensor as if the heat capacity of the sensor increased. This lowers the responsibility of the temperature sensor to changes in the temperature of the fixing member. As a result, when the temperature sensor senses the threshold temperature at which the current supply to the heater should be cut off, the actual surface temperature of the fixing member may have exceeded the threshold temperature.
The bearings and driving gears of the fixing and pressing members may be resinous. If dust sticks to the temperature sensor, so that the surface temperature of the fixing member rises excessively beyond a specified range, the resinous bearings and gears and other peripheral parts may thermally deform. If the surface temperature rises beyond this range, the fixing and pressing members may not fix a developer image well on a record medium.
JP-H10-143000-A discloses an apparatus for image formation, which includes a main power supply, a fixing member, a temperature sensor, and a controller. The temperature sensor senses the surface temperature of the fixing member. On the basis of the sensed temperature, the controller computes the thermonasty value of the surface temperature rising just after the power supply is switched on and the thermonasty value of the surface temperature dropping just after the supply is switched off. If either of the thermonasty values is outside a reference range, the controller determines that the temperature sensor or the power supply is abnormal.
It is impossible to detect an abnormality of the temperature sensor or the main power supply when the surface temperature of the fixing member neither rises just after the supply is switched on nor drops just after the supply is switched off. For example, if the apparatus is used with the power supply on for a long time, it may be impossible to detect an abnormality of the temperature sensor, except just after the supply is switched on, and except after the supply is switched off. This may make it impossible to detect an abnormality of the temperature sensor for a long time.
The controller of this apparatus needs to compute the thermonasty values, so that the load on the controller is heavy.
The object of the present invention is to provide an apparatus for image formation which prevents the surface temperatures of its fixing and pressing members from excessively rising, except just after the main power supply of the apparatus is switched on, and except after the supply is switched off.